1. The Movement of Fluid Across the Plasma Membrane

2. Describe the role of aquaporins in water movement across membranes.
Define and explain osmotic and hydrostatic forces.
Calculate the osmotic pressure gradient.
Define and explain tonicity.
Discuss the effect of hypertonic, isotonic and hypotonic -solutions on cell volume.
Define non-penetrating, rapid penetrating and slow penetrating solute
Describe the effect of the administration of various IV fluids on the internal environment.

3. The Movement of Water Across the Plasma Membrane
Water can move in and out of cells.
But the partition coefficient of water into lipids is low meaning the permeability of the membrane lipid bilayer for water is low.
Specific membrane proteins that function as water channels explain the rapid movement of water across the plasma membrane
These water channels are small integral membrane proteins known as aquaporins
ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium

4. Water Movement NaCl 0 mOsm NaCl 100 mOsm [water] HIGH [water] LOW
This diagram shows the passage of water molecules across a membrane that is
impermeable to solutes. In this situation water moves in the direction of low solute
concentration to high solute concentration (i.e. conceptually like high water concentration
to low water concentration).aquaporins allow for the passage of water
molecules across the membrane and so increase the permeability of the membrane to
water. This increases the net flux of water across the membrane.
Aquaporin
If membrane impermeable to NaCl

5. CLINICAL CORRELATION-
In the kidney, aquaporin-2 (AQP2) is abundant in the collecting duct and is the target of the hormone vasopressin, also known as antidiuretic hormone. This hormone increases water transport in the collecting duct by stimulating the insertion of AQP2 proteins into the apical plasma membrane. Several studies have shown that AQP2 has a critical role in inherited and acquired disorders of water reabsorption by the kidney.
For example, nephrogenic diabetes insipidus is a condition in which the kidney loses its ability to reabsorb water properly, resulting in excessive loss of water and excretion of a large volume of very dilute urine (polyuria). Although inherited forms of diabetes insipidus are relatively rare, it can develop in patients receiving chronic lithium therapy for psychiatric disorders, giving rise to the term lithium-induced polyuria.
Both of these conditions are associated with a decrease in the number of AQP2 proteins in the collecting ducts of the kidney.

6. Osmosis
- Osmosis is the flow of water across a semipermeable membrane from a solution with low solute concentration to a solution with high solute concen­tration.
The spontaneous movement of water across a membrane
driven by a gradient of water concentration is the process
known as osmosis.

7. The driving force for the movement of water across the plasma membrane is the difference in water concentration between the two sides of the membrane.
For historical reasons, this driving force is not called the chemical gradient of water but the difference in osmotic pressure.
The osmotic pressure of a solution is defined as the pressure necessary to stop the net movement of water across a selectively permeable membrane that separates the solution from pure water.
When a membrane separates two solutions
of different osmotic pressure, water will move from
the solution with low osmotic pressure (high water concentration)
to the solution of high osmotic pressure (low
water concentration). I

8. The Movement of Water Across the Plasma Membrane Is Driven by Differences in Osmotic Pressure
Osmotic pressure of a solution is defined as the pressure necessary to stop the net movement of water across a selectively permeable membrane
When a membrane separates two solutions of different osmotic pressure, water will move from –
the solution with low osmotic pressure (high water and low solute concentrations) to
the solution of high osmotic pressure (low water and high solute concentrations).

9. Solutions 1 and 2 are separated by a semipermeable membrane
Solutions 1 and 2 are separated by a semipermeable membrane. Solution 1 contains a solute that is too large to cross the membrane. Solution 2 is pure water. The presence of the solute in solution 1 produces an osmotic pressure.

10. g = number of particles in solution
The osmotic pressure of a solution can be calculated by -
Van't Hoffs law, which states that osmotic pressure depends on the concentration of osmotically active particles. The concentration of particles is converted to pressure according to the following equation:
where:
7T = osmotic pressure (mm Hg or atm)
g = number of particles in solution
R = gas constant (0.082 L-atm/mol-K)
σ = Reflection coefficient (varies from 0 to 1)
T = absolute temperature (K)
C = concentration (mol/L)
Osmotic pressure is determined solely by the number of molecules in that solution. It is not dependent on such factors as the size of the molecules, their mass, or their chemical nature (e.g., valence).

11. Reflection coefficient (σ)
is a number between zero and one that describes the ease with which a solute permeates a membrane.
a. If the reflection coefficient is 1, the solute is impermeable. Therefore, it is retained in the original solution, it creates an osmotic pressure, and it causes water flow. Serum albumin (a large solute) has a reflection coefficient of nearly one.
b. If the reflection coefficient is 0, the solute is completely permeable. Therefore, it will not exert any osmotic effect, and it will not cause water flow. Urea (a small solute) has a reflection coefficient of close to zero and it is, therefore, an ineffective osmole
Reflection coefficientis an index of the membrane’s permeability to the solute and varies between 0 and 1.
Particles that are impermeable to the membrane have a reflection coefficient of 1.
Particles that are freely permeable to the membrane have a reflection coefficient of 0.

12. Osmolarity refers to osmotic pressure generated by the dissolved solute molecules in 1L of solvent.
It depends strictly on the number of particles in solution (not the number of molecules, since some molecules (e.g. NaCl) dissociate into ions when in solution).
Osmolarity is therefore, the number of particles per liter of solution and is expressed in osmol/L or OsM or in the case of dilute solutions as milliosmol/L.
Ex- A solution of 1 M CaC12 has a higher osmotic pressure than a solution of 1 M KCl because the concentration of particles is higher.
The higher the osmotic pressure of a solution, the greater the water flow into it.

14. Units of concentration
mOsm (milliosmolar) or mOsm/L = an index of the concentration of particles per liter of solution
mM (millimolar) or mM/L = an index of the concentration of molecules dissolved per liter of solution
isotonic solutions = 300 mOsm = 150 mM NaCl (one NaCl molecule yields two particles in solution)
300 mOsm = 300 mM glucose

15. Osmolality and Tonicity
A solution’s osmolality is determined by the total concentration of all the solutes present.
In contrast, the solution’s tonicity is determined by the concentrations of only those solutes that do not enter(“penetrate”) the cell

16. Osmolarity
The 300 mOsm is rounded off from the true value of 285 to 290 mOsm.

17. Water flows from the hypo­tonic to the hypertonic solution.
Two solutions having the same effective osmotic pressure are isotonic because no water flows across a semipermeable membrane separating them.
If two solutions separated by a semipermeable membrane have different effective osmotic pressures, the solution with the higher effec­tive osmotic pressure is hypertonic and the solution with the lower effective osmotic pressure is hypotonic.
Water flows from the hypo­tonic to the hypertonic solution.
Tonicity describes a solution, and how that solution affects cell volume.
The tonicity of a solution depends not just on the osmolarity of the solution but also on whether the solutes (particles) in the solution are penetrating or not.

19. RBC hypotonic solution
Rules for predicting tonicity
If the cell has a higher concentration of non-penetrating solutes than the solution, there will be net movement of water into the cell. The cell swells, and by definition that solution is hypotonic.
SWELL
Diagram showing a RBC about to be dropped into a hypotonic solution. Using the above definition you would expect the RBC to swell in the hypotonic solution. 24

20. RBC isotonic solution NO VOLUME CHANGE
If the concentrations of non-penetrating solutes are the same inside and in the solution, there is no net movement of water at equilibrium and the solution is isotonic.
NO VOLUME CHANGE

21. RBC hypertonic solution
If the cell has a lower concentration of non-penetrating solutes than the solution, there is net movement of water out of the cell, the cell shrinks, and the solution is hypertonic
SHRINK

22. Tonicity
Tonicity describes the volume change of a cell placed in a solution

23. Problems involving a non penetrating solute
Predict the changes in cell volume (increase, decrease, no change) when a normal RBC previously equilibrated in isotonic saline is placed in the following solutions.
Assume the fluid volume of the external solution is large, and thus, as water moves in or out of the cell, there is no significant change in the concentration of beaker solutes .
mOsm NaCl
mOsm NaCl
mM NaCl
mM NaCl
Answers
mOsm NaCl: Because the effective osmolarity of the solution is <300
mOsm, the RBC will swell. Cells in hypotonic saline swell.
mOsm NaCl: Because the effective osmolarity of the solution is >300
mOsm, the RBC will shrink. Cells in hypertonic saline shrink.
mM NaCl: This is equivalent to 300 mOsm NaCl or isotonic saline.
There is no change in RBC volume.
mM NaCl: This is equivalent to 600 mOsm NaCl or hypertonic saline.
Cells in hypertonic saline shrink.

24. Effect of substances that rapidly penetrate cell membranes
The presence of a substance, such as urea, ,5% dextrose that penetrates the cell membrane quickly does not affect the osmotic movement of water.
If the total concentration of non penetrating solutes is <300 mOsm, the RBC will swell; if it is >300 mOsm,the RBC will shrink.

25. Problems involving a rapidly penetrating solute
Predict the changes in cell volume (increase, decrease, no change) when a normal RBC previously equilibrated in isotonic saline is placed in the following solutions:
200 mOsm NaCl and 200 mOsm urea
300 mOsm urea only
500 mOsm urea only
mOsm NaCl and 200 mOsm urea: The effective osmolarity of the
solution is determined only by the nonpenetrating solutes. A penetrating
substance, such as urea, will diffuse across the membrane and equalize its
concentration in the two compartments. Therefore, it will not contribute
to effective osmolarity. If the effective osmolarity is less than 300, the cell
swells. Here the effective osmolarity is 200; therefore, the cell swells.
mOsm urea only: The effective osmolarity of the solution is zero, which
is the same as pure water; therefore, the cell swells.
mOsm urea only: Again, the effective osmolarity is zero; therefore, the
cell swells.

26. Effect of substances that slowly penetrate cell membranes
Some substances( glycerol) penetrate cell membranes but do so slowly.
Thus, they initially have an osmotic effect like sodium chloride but no osmotic effect at equilibrium.
Problem involving a slowly penetrating solute
Q.Predict the changes in cell volume (increase, decrease, no change) when a normal RBC previously equilibrated in isotonic saline is then placed in the following solution. Determine the initial effect versus the long-term effect.
200 mOsm NaCl and 200 mOsm glycerol (a slowly penetrating substance)
200 mOsm NaCl and 200 mOsm glycerol (a slowly penetrating substance):
Timing is important in this question. Initially, the glycerol will not penetrate;
therefore, it contributes to the initial effective osmolarity of the solution.
Because the initial effective osmolarity is 400, the cell will shrink. With time,
the glycerol will penetrate the membrane and equalize its concentration in
the two compartments. The long-term effective osmolarity will be due to
only the NaCl, 200 mOsm. Therefore, over the long term, the cell will swell.

28. The Clinical Relevance of Understanding Tonicity
The importance of understanding this well is to make sure that you understand the basis and rationale for intravenous fluid therapy.
Several IV fluids exist e.g.
0.9% saline(normal saline)
5% dextrose in normal saline
5% dextrose in water
half normal saline
5% dextrose in half normal saline. (Dextrose is glucose).
How does the clinician decide which fluid to use? Well, it depends on what the objectives are – replacement of blood volume or rehydration of cells in dehydrated individuals.

29. Discussion on IV solutions
First thing to do is to look at the relative osmolarity and tonicity of the solution to the extracellular (and intracellular) fluid. Then take into account what effect this will have on the volumes of the two fluid compartments.
% saline. This has the same osmolarity as the intracellular fluid. The saline is NaCl so the two particles Na and Cl are considered to be non-penetrating. Therefore this solution is iso-osmotic and isotonic. Because it is isotonic it will not change the tonicity of the extracellular fluid and so the extracellular fluid will remain isotonic to the intracellular fluid. Therefore NO FLUID MOVEMENT INTO THE CELLS. This solution would be suitable for replacing blood (extracellular fluid).

30. 5% Dextrose in normal saline
5% Dextrose in normal saline. 5% dextrose is iso-osmotic to the intracellular fluid, so is normal saline. Therefore you must take into account both of these when working out the overall osmolarity. This solution has twice the osmolarity of the intracellular fluid. Therefore it is HYPEROSMOTIC. Dextrose is penetrating , so makes no contribution to the tonicity of the solution. Saline is non-penetrating it does make a contribution. Therefore the solution is ISOTONIC. Infusion of this solution into the veins would not change the tonicity of the extracellular fluid so NO NET FLUID MOVEMENT INTO THE CELLS. Notice the NET. Rapid infusion of this solution will lead initially to some water movement out of the cells which will be reversed as the dextrose moves into the cells. This solution would be suitable for replacing blood.
5% Dextrose in water. 5% dextrose is iso-osmotic to the intracellular fluid. Water is, of course, hypo-osmotic (0 mosm)(it has no particles). This solution therefore is iso-osmotic to the intracellular fluid. Water has no tonicity and dextrose is penetrating . This solution has no tonicity so is HYPOTONIC to the intracellular fluid. Infusion of this solution will make the extracellular fluid hypotonic to the intracellular fluid so some of the infused fluid will enter the cell. THERE IS THEREFORE FLUID MOVEMENT INTO THE CELLS. This solution would be suitable for rehydrating cells.

31. Half normal saline. This is 0
Half normal saline. This is 0.45% saline, so has half the number of particles as normal saline so it is hypo-osmotic to the intracellular fluid. The particles are non-penetrating but again you have half the number of particles. This solution is therefore HYPOTONIC to the intracellular fluid. Infusion of this solution will make the extracellular fluid hypotonic to the intracellular fluid so some of the infused fluid will enter the cell. THERE IS THEREFORE FLUID MOVEMENT INTO THE CELLS. This solution would be suitable for rehydrating cells.
5% Dextrose in half normal saline. 5% dextrose is iso-osmotic to the intracellular fluid, half normal saline is hypo-osmotic. However if you have them together in a solution the result is hyperosmotic to the intracellular fluid. Only the saline is non-penetrating , therefore the solution is HYPOTONIC to the intracellular fluid. Infusion of this fluid will decrease the tonicity of the extracellular fluid. THERE IS THEREFORE FLUID MOVEMENT INTO THE CELLS. This solution would be suitable for rehydrating cells.

32. In order to replenish the fluid and electrolyte loss in diarrhoea, a person is given ORS. Of the following composition of ORS, which of the following along with Na is more important for replenishing Na loss? K Cl
Glucose
Citrate

33. Oral Rehydration Therapy Is Driven by Solute Transport
Oral administration of rehydration solutions has dramatically reduced the mortality resulting from cholera and other diseases that involve excessive losses of water and solutes from the gastrointestinal tract. The main ingredients of rehydration solutions are glucose, NaCl, and water. The glucose and Na+ ions are reabsorbed by SGLT1 and other transporters in the epithelial cells lining the lumen of the small intestine .
Deposition of these solutes on the basolateral side of the epithelial cells increases the osmolarity in that region compared with the intestinal lumen and drives the osmotic absorption of water. Absorption of glucose, and the obligatory increases in absorption of NaCl and water, helps to compensate for excessive diarrheal losses of salt and water.